Stellar P larimetry in .NET

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Stellar P larimetry
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spectrum (see Sten o, 1996 and Sten o & Keller, 1997) Arago (see Arago, 1855c and Grant, 1852) is also credited with the discovery that cometary light (namely the comets of 1819 and 1835 (Comet Halley)) is polarized Unlike astrometry, photometry and spectroscopy, the advances of polarimetry in the nineteenth century were relatively slow being limited to the Moon, this being of interest only because of its extreme brightness accompanied with high levels of polarization Notable contributions in the nineteenth century were by Secchi (1860) and Lord Rosse (see Parsons 1878) The key works related to lunar polarimetry from this period and the early part of the twentieth century have been referenced by Fielder (1961) and also described by Turner (1957, 1958) It may also be mentioned that the Moon s light, particularly from the dark maria at a phase 110 , provides a simple, but delicate, opportunity to observe polarization directly by eye by rotating a polarizer before it, with, or without, the use of a telescope Planetary work was also initiated at the turn of the century by Lyot (1929) through the ingenious design of a sensitive visual polarimeter His observations were seminal, acting as reference for a whole range of later measurements of the Moon, planets, asteroids and rough scattering by laboratory samples One of the highlights resulting from a development of this work is the determination of asteroid diameters by polarimetry According to Umov (1912), the albedo of a rough surface is inversely proportional to the amount of polarization in the scattered light With good calibration, partly obtained by laboratory measurements, and partly through telescope observations, polarimetry of asteroids over their phase angle range provides albedo values For any asteroid, photometric measurement of its absolute magnitude, together with its polarimetric albedo, then allows a cross-sectional area to be determined, from which a diameter is obtained Early work in this area was undertaken by Bowell & Zellner (1974)
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13 The Dawning of Stellar Polarimetry
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The rst attempt to measure polarization in the light from stars appears to have been made by hman (1934) when he used a photographic technique to investigate possible polarimetric variations within spectral lines of the famous eclipsing binary, Lyr At the time, tentative claims were made for positive results, but on re ection, some 30 years later, hman (1965) commented that he was now more cautious about his earlier detection levels and that the photographic method was probably not suf ciently sensitive to give conclusive answers in every respect Encouragement to publish his results was offered by the Editor of the scienti c journal Nature, following a visit by him and his wife to Stockholm Observatory The key paper which triggered observational activity in stellar polarimetry was that of Chandrasekhar (1946) who predicted that the continuous radiation of earlytype stars should be polarized By considering the opacity of the atmospheres of such stars to be the result of electron scattering, he demonstrated that the radiation
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1 Introduction and History
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emerging from the stellar limb would have a polarization of just over 11%, with the azimuth of vibrations tangential to the limb, the polarization becoming zero for radiation emerging from the centre of the disc Quoting from his paper, he says:
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It is not impossible that this predicted polarization of the radiation of the early-type stars (in which scattering by free electrons is believed to play an important part in the transfer of radiation) would be detected under suitably favourable conditions
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Radial symmetry rules out there being a net polarization in the global radiation, but for an eclipsing binary in which a larger late-type companion partially masks the disc of the early-type star, the symmetry is broken and a polarization modulation is to be expected during the eclipse phase It is of interest to note that, at about the same time as Chandrasekhar s work on radiative transfer, Kopal & Shapley (1946) suggested that polarization modulations might be expected in such stars as V 444 Cyg, comprising a Wolf Rayet and O star eclipsing system, embedded in an extended dissociated atmosphere of free electrons As well as potentially revealing polarization by breaking the symmetry as consequence of eclipses, hman (1946) demonstrated by qualitative argument that, for stars with high values of v sin i, a variation of polarization might be seen at all times across the Doppler rotationally broadened pro les, with the wings of the lines being weighted by radiation from the equatorial limb and the line core being weighted by light from the centre of the stellar disc At the time of this proposal, measurement techniques were insuf cient to explore the proposition
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